Automatic identification systems that include identification devices (also called data carriers, escort memories, cards, or ID tags) that deliver information from the identification devices by coding, modulation and reflection of an incident microwave signal in the form of an information containing sideband, without supplying further energy to the identification device, have been known in the art since the middle of the 1970s. Such RFID systems (Radio Frequency Identification systems) are related to “backscatter technology” and are particularly common in the microwave range, e.g. at 915 MHz, 2.45 GHz and 5.8 MHz. 2.45 GHz is also a frequency band that can be used for other radiation sources, such as RFDC links, for instance (Radio Frequency Data Communication links). This means that interference problems originating from RFDC systems can occur.
The present invention provides an RFID system that is robust with respect to interference from, among others, these RFDC systems, but also with respect to interference from other systems which give rise to interferences that are short and unsynchronized in relation to the identification messages.
As distinct from RFID, RFDC operates with microwave transmitters at both ends of the transmission link. RFDC systems often utilize so called spread spectrum technology, where the RFDC messages are transmitted at different microwave frequencies in accordance with a controlled pattern in order to counteract fading effects and interferences/disturbances, from other systems, and where the frequency hops rapidly between different values, so-called frequency hopping.
RFID systems can also operate in accordance with spread spectrum technology, and the invention can thus also be applied to minimize interferences from such systems, and also to suppress interferences from any other system which gives rise to interferences that are shorter than the identification messages of the RFID system.
One problem with present day identification systems is that they are highly sensitive to interferences from sources that deliver signals at the receiver frequency of the RFID systems, for instance in the 2.45 GHz band, which is freely available for many applications. Examples of interference sources include so-called Bluetooth links, systems according to the 802.11 standard, RFID systems that include so called downlinks (i.e. systems that send short microwave pulses to the identification device in order to activate and/or to send data to the identification device), RFID systems that include battery-free identification devices, which are energized from the microwave transmitter of the reader via a microwave signal that is modulated in some way or another, radar stations, microwave ovens, personal detectors in alarm systems, automatic door openers, video transmission links, etc.